Thin film resonators (TFR's) have been and are currently being investigated as feedback elements in rf/microwave frequency oscillators. (See, for example, "UHF Oscillator Performance Using Thin Film Resonator Based Topologies", by Burns et al., 41st Annual Frequency Control Symposium--1987, pp. 382-387.) One form of thin film resonator is the overmoded stacked crystal filter (SCF) which has been investigated as the feedback element in an oscillator to produce an oscillator output in the form of a comb of frequencies normally associated with the overmoded stacked crystal filter itself. Such a device is described in "Design and Performance of Oscillators Using Semiconductor Delay Lines" by Burns et al., 1987 Ultrasonics Symposium, pp. 369-373. Insofar as applicants are aware, investigation of thin film resonators in oscillators, and particularly in stacked crystal filter oscillators, has not heretofore progressed to the stage where automatic electronic tuning of individual frequencies has been accomplished.
In other contexts, however, voltage control of oscillators has proven to be useful. Voltage control is accomplished by utilizing means such as a varactor in conjunction with an oscillator of the type normally considered tunable, with the varactor being the control element which serves to tune the oscillator to a frequency related to the voltage applied to the varactor.
Insofar as applicants are aware, and at least in large part because of the many complexities involved, the voltage control techniques have not been applied to thin film resonators particularly those of the stacked crystal filter variety.